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          <h1 class="post-title" itemprop="name headline">JVM类加载器</h1>
        

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        <p>在类加载的阶段，虚拟机是通过类的全限定名来获取描述该类的二进制字节流。那么，虚拟机是怎么通过了类的全限定名来获<br>取类的二进制字节流。其实，获取类的二进制字节流的这个动作在Java 虚拟机的外部实现，以便让应用程序自身决定如何获取<br>所需要的类，而实现这个动作的模块就是 “类加载器”。<br><a id="more"></a></p>
<h4 id="一、类与类加载器"><a href="#一、类与类加载器" class="headerlink" title="一、类与类加载器"></a>一、类与类加载器</h4><p>类加载器虽然只用于实现类的的加载，但作用却远远不限于类的加载阶段。对于任意的一个类，要确定其唯一性，就需要加载这<br>个类的类加载器和这个类本身一同确定。换句话说，要比较两个类是否”相等”,前提是两个类是由同一个类加载器加载的；否则，<br>即时两个类来源于同一个 Class 文件，被同一个虚拟机加载，只要加载它们的类加载器不同,这两个类就不”相等”。这里的相<br>等是指包括代表类的 Class对象的 equals() 方法，isAssignableFrom()方法、isInstance() 方法的返回结果，还包<br>括 instanceOf 关键字判断对象所属关系等情况。<br>eg:<br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">ClassLoaderTest</span> </span>&#123;</span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> <span class="keyword">throws</span> Exception </span>&#123;</span><br><span class="line">    ClassLoader classLoader = <span class="keyword">new</span> ClassLoader() &#123;</span><br><span class="line">     <span class="meta">@Override</span></span><br><span class="line">     <span class="keyword">public</span> Class&lt;?&gt; loadClass(String name) <span class="keyword">throws</span> ClassNotFoundException &#123;</span><br><span class="line">       <span class="keyword">try</span> &#123;</span><br><span class="line">         String fileName = name.substring(name.lastIndexOf(<span class="string">"."</span>)+<span class="number">1</span>) + <span class="string">".class"</span>;</span><br><span class="line">         InputStream is = getClass().getResourceAsStream(fileName);</span><br><span class="line">         <span class="keyword">if</span>(is == <span class="keyword">null</span>) &#123;</span><br><span class="line">           <span class="keyword">return</span> <span class="keyword">super</span>.loadClass(name);</span><br><span class="line">         &#125;</span><br><span class="line">         <span class="keyword">byte</span>[] b = <span class="keyword">new</span> <span class="keyword">byte</span>[is.available()];</span><br><span class="line">         is.read(b);</span><br><span class="line">         <span class="keyword">return</span> defineClass(name,  b, <span class="number">0</span>, b.length);</span><br><span class="line">       &#125; <span class="keyword">catch</span> (IOException e) &#123;</span><br><span class="line">         <span class="keyword">throw</span> <span class="keyword">new</span> ClassNotFoundException(name);</span><br><span class="line">       &#125;</span><br><span class="line">     &#125;</span><br><span class="line">    &#125;;</span><br><span class="line">    Object object = classLoader.loadClass(<span class="string">"ClassLoaderTest"</span>).newInstance();</span><br><span class="line">    System.out.println(object.getClass());  <span class="comment">// class ClassLoaderTest</span></span><br><span class="line">    System.out.println(object <span class="keyword">instanceof</span> ClassLoaderTest);  <span class="comment">// false</span></span><br><span class="line">  &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure></p>
<h4 id="二、双亲委派模型"><a href="#二、双亲委派模型" class="headerlink" title="二、双亲委派模型"></a>二、双亲委派模型</h4><p>从虚拟机上看，只会有两种不同的类加载器：<br>①. 启动类加载器（Bootstrap ClassLoader）:这个类加载使用的 C++实现，是虚拟机自身的一部分。<br>②. 其他的类加载器，都由Java语言实现，独立于虚拟机，并且全部继承于 java.lang.ClassLoader。<br>在开发人员看来，绝大部分的Java 程序都会使用到以下3中系统提供的类加载器。</p>
<ol>
<li><p>启动类加载器（Bootstrap ClassLoader）：这个类加载器负责将存放在 &lt;JAVA_HOME&gt;\lib目录中或者被-Xbootclasspath<br>参数所指定的路径中的，并且是虚拟机识别的类库加载到虚拟机内存中。启动类加载器无法被Java程序直接引用，如果用户在自定义类加<br>载器时，需要把加载请求委派给引导类加载器，那直接使用 null 代替即可。<br>eg:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> ClassLoader <span class="title">getClassLoader</span><span class="params">()</span> </span>&#123;</span><br><span class="line">    ClassLoader cl = getClassLoader0();</span><br><span class="line">    <span class="keyword">if</span> (cl == <span class="keyword">null</span>)&#123;</span><br><span class="line">       <span class="keyword">return</span> <span class="keyword">null</span>;</span><br><span class="line">    &#125;</span><br><span class="line">    SecurityManager sm = System.getSecurityManager();</span><br><span class="line">    <span class="keyword">if</span>(sm != <span class="keyword">null</span>)&#123;</span><br><span class="line">       ClassLoader ccl = ClassLoader.getCallerClassLoader();</span><br><span class="line">       <span class="keyword">if</span>(ccl != <span class="keyword">null</span> &amp;&amp; ccl != cl &amp;&amp; !cl.isAncestor())&#123;</span><br><span class="line">            ClassLoader.checkClassLoaderPermission(cl, Reflection.getCallerClass());</span><br><span class="line">       &#125;</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="keyword">return</span> cl;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
</li>
<li><p>扩展类加载器(Extension ClassLoader):这个加载器由 sun.misc.Launcher$ExtClassLoader 实现。它负责加载&lt;JAVA_HOME&gt;\lib\ext 目录中、或者被java.ext.dirs 系统变量所指定的路径中的所有类库，<br>开发人员可以直接使用扩展类加载器。</p>
</li>
<li>应用程序加载器（Application Class Loader）：这个类加载器由  sun.misc.Launcher$AppClassLoader 实现，<br>由于这个类加载器是Class Loader 中的 getSystemClassLoader() 方法的返回值，所以也被称为系统类加载器。它负责加载用户类<br>路径上所指定的类库。如果程序中没有定义过自己的类加载器，一般情况下这个类加载器就是默认的类加载器。</li>
</ol>
<p>如下图，展示了类加载器之间的层次关系，这种关系称为类加载器的双亲委派模型。<br><!-- ![image](/images/jvm-class-loader/双亲委派模型.png) --><br><img src="/2018/11/18/2018-11-18-jvm-class-loader/双亲委派模型.png" alt="双亲委派模型">   </p>
<p>双亲委派模型要求除了顶层的启动类加载器之外，其余的类加载器都应当有自己的父类加载器。这里的类加载器之间的父子关系一般不会以继承的关系来实现，而是以组合的关系来复用父加载器的代码。双亲委派模型的工作过程：如果一个类加载器收到了类加载的请求，它首先不会自己去尝试加载这个类，而是把这个请求委派给自己的父类加载器去完成，每一个层次的类加载器都是如此，因此所有的加载请求最终都应该是传送到顶层的启动类加载器中，只有当父加载器反馈自己无法完成这个加载请求(它的搜索范围中没有找到所需要的类)时，子加载器才会尝试自己去加载。使用双亲委派模型组织类加载之间的关系有一个好处就是：Java类随着它的类加载器一起具备了一种带有优先级的层次关系。例如 java.lang.Object 类，无论哪一个类加载器去加载这个类，最终都是委派给处于模型最顶端的启动类加载器去加载，因此，Object类在程序的各类加载器环境中都是同一个类。</p>
<p>双亲委派模型的代码：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">protected</span> Class&lt;?&gt; loadClass(String name, <span class="keyword">boolean</span> resolve)                           </span><br><span class="line"><span class="keyword">synchronized</span> (getClassLoadingLock(name)) &#123;                                       </span><br><span class="line">    <span class="comment">// First, check if the class has already been loaded                         </span></span><br><span class="line">    Class&lt;?&gt; c = findLoadedClass(name);                                          </span><br><span class="line">    <span class="keyword">if</span> (c == <span class="keyword">null</span>) &#123;                                                             </span><br><span class="line">        <span class="keyword">long</span> t0 = System.nanoTime();                                             </span><br><span class="line">        <span class="keyword">try</span> &#123;                                                                    </span><br><span class="line">            <span class="keyword">if</span> (parent != <span class="keyword">null</span>) &#123;                                                </span><br><span class="line">                c = parent.loadClass(name, <span class="keyword">false</span>);                               </span><br><span class="line">            &#125; <span class="keyword">else</span> &#123;                                                             </span><br><span class="line">                c = findBootstrapClassOrNull(name);                              </span><br><span class="line">            &#125;                                                                    </span><br><span class="line">        &#125; <span class="keyword">catch</span> (ClassNotFoundException e) &#123;                                     </span><br><span class="line">            <span class="comment">// ClassNotFoundException thrown if class not found                  </span></span><br><span class="line">            <span class="comment">// from the non-null parent class loader                             </span></span><br><span class="line">        &#125;                                                                        </span><br><span class="line">        <span class="keyword">if</span> (c == <span class="keyword">null</span>) &#123;                                                         </span><br><span class="line">            <span class="comment">// If still not found, then invoke findClass in order                </span></span><br><span class="line">            <span class="comment">// to find the class.                                                </span></span><br><span class="line">            <span class="keyword">long</span> t1 = System.nanoTime();                                         </span><br><span class="line">            c = findClass(name);                                                 </span><br><span class="line">            <span class="comment">// this is the defining class loader; record the stats               </span></span><br><span class="line">            sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);     </span><br><span class="line">            sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);      </span><br><span class="line">            sun.misc.PerfCounter.getFindClasses().increment();                   </span><br><span class="line">        &#125;                                                                        </span><br><span class="line">    &#125;                                                                            </span><br><span class="line">    <span class="keyword">if</span> (resolve) &#123;                                                               </span><br><span class="line">        resolveClass(c);                                                         </span><br><span class="line">    &#125;                                                                            </span><br><span class="line">    <span class="keyword">return</span> c;                                                                    </span><br><span class="line">&#125;                                                                                </span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure></p>

      
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